Power flow controller with a fractionally rated back-to-back converter

ABSTRACT

A power flow controller with a fractionally rated back-to-back (BTB) converter is provided. The power flow controller provide dynamic control of both active and reactive power of a power system. The power flow controller inserts a voltage with controllable magnitude and phase between two AC sources at the same frequency; thereby effecting control of active and reactive power flows between the two AC sources. A transformer may be augmented with a fractionally rated bi-directional Back to Back (B TB) converter. The fractionally rated BTB converter comprises a transformer side converter (TSC), a direct-current (DC) link, and a line side converter (LSC). By controlling the switches of the BTB converter, the effective phase angle between the two AC source voltages may be regulated, and the amplitude of the voltage inserted by the power flow controller may be adjusted with respect to the AC source voltages.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/673,966, filed on Nov. 9, 2012, entitled “Power Flow Controller witha Fractionally Rated Back-to-Back Converter,” which claims the benefitof U.S. Provisional Patent Application No. 61/558,706, filed on Nov. 11,2011, entitled “Power Flow Controller with a Fractionally RatedBack-to-Back Converter,” which are incorporated by reference herein intheir entireties.

STATEMENT OF RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH

This invention was made with Government support under DE-AR0000108awarded by the United States Department of Energy. The Government hascertain rights in the invention.

TECHNICAL FIELD

The present invention(s) relate generally to controlling power flow inan electric power system. More particularly, the invention(s) relate topower flow controllers with back-to-back converters.

DESCRIPTION OF THE RELATED ART

An electric power system is a network of interconnected electricalequipment that generates, transmits, and consumes electric power.Electric power is delivered to consumers through a transmission networkand a distribution network from generators to consumers. Thetransmission network and the distribution network are often known as thetransmission grid and the distribution grid, respectively. Operation ofthe transmission grid and the distribution grid was once straightforwardbefore the deregulation of the electric power market, but becameextremely complex as a result of the competition among various utilitycompanies. Increased amount of electric power is flowing in the electricpower system and causing congestion and overflow in certain parts of theelectric power system, which may limit the capacity and also impact thereliability of the electric power system. As the electric power systemis highly dynamic, real-time power flow control ensures the electricpower system's reliability and increases its capacity and efficiency. Asa result, the increasing load demand, increasing level of penetration ofrenewable energy and limited transmission infrastructure investmentshave significantly increased the need for a smart dynamicallycontrollable grid.

Traditionally, power flow control has been achieved by generatorcontrol, shunt VAR compensation and LTC tap settings. However, the rangeof control achieved is not very significant and the dynamic response isvery slow. Various devices can be installed on the electric power systemto perform electric power flow controls such as a Phase Angle Regulator(PAR), also known as a Phase Shifting Transformer (PST), a Unified PowerFlow Controller (UPFC), and a Back-to-Back (BTB) HVDC link.

PARs or PSTs correct the phase angle difference between two parallelconnected electrical transmission systems and thereby control the powerflow between the two systems so that each can be loaded to its maximumcapacity. Conventional PARs and PSTs insert a series voltage to a phasethat is in quadrature with the line-to-neutral voltage. However,conventional PARs or PSTs cannot control the reactive power flowindependently from the active power flow. Their dynamic capabilities, ifthey exist, are also very limited. UPFCs consist of two inverters withan intermediate DC bus with energy storage. One inverter is connected inshunt through transformer, while the second inserts a series voltage inthe line, again through transformer coupling. UPFCs typically can inserta desired series voltage, balancing average power flow using the shuntinverter. This allows a UPFC to source or sink active and reactivepower. UPFCs are typically used at very high power and voltage levels(100 MW @ 345 KV). The need for UPFCs to survive faults and abnormalevents on the grid makes their design complex and expensive because theseries transformers and inverters for operation under system faults arelarge and expensive. Moreover, the shunt transformer and inverters foroperation under transient voltages also add cost. As a result, althoughUPFCs have been commercially available for decades, few have beendeployed.

BTB HVDC links consist of two inverters with an intermediate DC bus withenergy storage. BTB HVDC links provide a wide control range (+/−1 p.u.)for both active and reactive power. However, for a 1 p.u. control range,the converter has to be rated for at least 2 p.u. (two converters of 1p.u. each). Building such high power controllers for transmission orsub-transmission systems is extremely complex and expensive. Also, thesize and complexity may affect their reliability. As the two invertersare connected in series, effectively a single point of failure in thesystem is created.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

According to various embodiments of the invention, a power flowcontroller with a fractionally rated back-to-back (BTB) converter isprovided. Various embodiments provide dynamic control of both active andreactive power of a power system. The power flow controller inserts avoltage with controllable magnitude and phase between two AC sources;thereby effecting control of active and reactive power flows between thetwo AC sources. In one embodiment, a transformer is augmented with afractionally rated bi-directional Back to Back (BTB) converter. Invarious embodiments, low-rating insulated gate bipolar transistors(IGBTs) are used as switches in the fractionally rated converters.Further, a power flow controller may be isolated from a system fault. Insome embodiments, a fail-normal switch bypasses the power flowcontroller in case of a contingency.

A power flow controller with a fractionally rated BTB converter providescontrol of both the active and reactive power flow between two ACsources at the same frequency. In various embodiments, the fractionallyrated BTB converter comprises a transformer side converter (TSC), adirect-current (DC) link, and a line side converter (LSC). Bycontrolling the switches of the BTB converter, the effective phase anglebetween the two AC source voltages may be regulated, and the amplitudeof the voltage inserted by the power flow controller may be adjustedwith respect to the AC source voltages. Various embodiments may beimplemented at various voltage levels such as 13 kV, 69 kV, and 139 kV.Further, in some embodiments, a power flow controller comprises afail-normal switch. As the fault current is diverted by the bypassswitch until line breakers trip, the transformer and the BTB converterof the power flow controller are isolated from fault currents or hightransient voltages during any fault.

Other features and aspects of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresin accordance with embodiments of the invention. The summary is notintended to limit the scope of the invention, which is defined solely bythe claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention, in accordance with one or more variousembodiments, is described in detail with reference to the followingfigures. The drawings are provided for purposes of illustration only andmerely depict typical or example embodiments of the invention. Thesedrawings are provided to facilitate the reader's understanding of theinvention and shall not be considered limiting of the breadth, scope, orapplicability of the invention. It should be noted that for clarity andease of illustration these drawings are not necessarily made to scale.

FIG. 1 illustrates an exemplary system diagram of an electric powersystem where various embodiments of the invention can be implemented.

FIG. 2 illustrates an exemplary schematic diagram of a single-phasepower flow controller in accordance with an embodiment of the invention.

FIG. 3 illustrates an exemplary schematic diagram of a single-phase3-level power flow controller in accordance with an embodiment of theinvention.

FIG. 4A is a diagram illustrating a system with an installation of apower flow controller in accordance with an embodiment of the invention.

FIG. 4B is a vector diagram illustrating principles of operation of apower flow controller in accordance with an embodiment of the invention.

FIGS. 5A-C illustrate simulation waveforms of an embodiment of theinvention as described herein.

FIG. 6A depicts common mode and differential mode controls implementedin various embodiments of the invention as described herein.

FIG. 6B-D illustrate control block diagrams of various embodiments ofthe invention as described herein.

FIG. 7 illustrates an example computing module that may be used inimplementing various features of embodiments of the invention.

The figures are not intended to be exhaustive or to limit the inventionto the precise form disclosed. It should be understood that theinvention can be practiced with modification and alteration, and thatthe invention be limited only by the claims and the equivalents thereof.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

According to various embodiments of the invention, a power flowcontroller with a fractionally rated back to back (BTB) converter isprovided. Various embodiments provide dynamic control of both active andreactive power of a power system. The power flow controller inserts avoltage with controllable magnitude and phase between two AC sources;thereby effecting control of active and reactive power flows between thetwo sources. In one embodiment, a transformer is augmented with afractionally rated bi-directional BTB converter. In various embodiments,switches of the BTB converter may be implemented by low-rating insulatedgate bipolar transistors (IGBTs). Further, the power flow controller maycomprise a fail-normal switch that bypasses the power flow controller incase of a contingency. As the fault current is diverted by the bypassswitch until line breakers trip, the transformer and the BTB converterof the power flow controller are isolated from fault currents or hightransient voltages during any fault.

A power flow controller with a BTB converter controls both the activeand the reactive power flow between two AC sources at the samefrequency. In various embodiments, the fractionally rated BTB convertercomprises a transformer side converter (TSC), a direct-current link, anda line side converter (LSC). By controlling the switches in thefractionally rated converter, the effective phase angle between the twovoltages may be regulated and the amplitude of the voltage inserted bythe power flow controller may be adjusted with respect to the AC sourcevoltages.

Before describing the invention in detail, it is useful to describe afew example environments with which the invention can be implemented.One such example is that of illustrated in FIG. 1.

FIG. 1 illustrates an exemplary system diagram of an electric powersystem 100 where various embodiments of the invention can beimplemented. The electric power system 100 comprises generators 101 and102; loads 110 and 111; and transmission lines 103-107, which may havedifferent ratings and are loaded differently. Various power flowcontrollers may be deployed to the power system 100. In the illustratedexample, two power flow controllers 108 and 109 are installed. As aresult of this installation, power flows of the power system 100 may becontrolled. In other words, both the active and reactive power alongeach transmission line of the power system 100 may be redirected.

From time-to-time, the present invention is described herein in terms ofthis example environment. Description in terms of these environments isprovided to allow the various features and embodiments of the inventionto be portrayed in the context of an exemplary application. Afterreading this description, it will become apparent to one of ordinaryskill in the art how the invention can be implemented in different andalternative environments.

FIG. 2 illustrates an exemplary schematic diagram of a single-phasepower flow controller 200 in accordance with an embodiment of theinvention. The example power flow controller 200 comprises a transformer201 and a BTB converter 230. In one embodiment, the transformer 201 maybe an autotransformer. In one embodiment, the transformer 201 may be anisolated step-down transformer. The BTB converter 230 comprises atransformer side converter (TSC) 202 which is an AC-DC converter, adirect-current (DC) link 203, a line side converter (LSC) 204 which is aDC-AC converter, input filter inductors 215 and 216, and output filterinductors 217 and 218. The LSC 204 may be a full-bridge or a half-bridgeDC-AC converter. The TSC 202 and the LSC 204 are connected through thecommon DC link 203. In the illustrated example, the common DC link 203comprises a capacitor 205 and is a capacitor link. The TSC 202 comprisesswitches 206-209, and the LSC comprises switches 210-213. The TSC 202comprises legs 220 and 221: the leg 220 comprises switches 206 and 208,and the leg 221 comprises switches 207 and 209. The legs 220 and 221 ofthe TSC 202 may be coupled between two taps 241 and 242 of thetransformer 201. In one embodiment, the legs 220 and 221 are coupledbetween taps +/−n of the transformer 201, respectively. Further, the LSC204 comprises legs 222 and 223: the leg 222 comprises switches 210 and212 and the leg 223 comprises switches 211 and 213. The legs of the LSC204 may be both coupled to the output of the BTB converter 230, eitherdirectly or indirectly.

The power flow controller 200 may be installed between two AC twosources. In one embodiment, the power flow controller 200 may beinstalled in series with a transmission line between the two AC sources.In various embodiments, taps of the transformer 201 may be floatingaround the voltage level of the transmission line where the power flowcontroller is installed. The input of the power flow controller 200 isconnected to one node of a transmission line and the output of the powerflow controller 200 is connected to another node of the transmissionline. In various embodiments, the mid-point of the BTB converter 230 mayserve as the input of the power flow controller 200. In furtherembodiments, the tap 240 of the transformer 201 may serve as the inputof the power flow controller 200. Taps of the transformer 201 may befloating around the voltage level at the tap 240 of the transformer 201,which may be the line voltage. The legs 220 and 221 of the TSC 202 maybe coupled between taps 241 and 242 of the transformer 201,respectively, either directly or indirectly. The tap 240 may be a tapthat is located between the taps 241 and 242. In one embodiment, thetaps 241 and 242 are +/−n of the transformer 201, and the tap 240 is thecenter tap of the tap winding (i.e., +/−n) of the transformer 201. Invarious embodiments, the output of the power flow controller 200 is theoutput of the BTB converter 230. The output of the power flow controller200 is coupled to another node of the transmission line.

The control module 220 regulates the switches 206-213. In variousembodiments, the control module 220 may generate switching pulses toregulate the turn-on and turn-off of each switch. In some embodiments,the control module 220 may interact with the gate drivers for switches206-213. In various embodiments, the switches 206-213 are two-quadrantswitches that conduct currents in both directions but may block voltagesin one direction. In some embodiments, insulated-gate bipolartransistors (IGBTs) with an antiparallel diode ormetal-oxide-semiconductor field-effect transistor (MOSFETs) with anantiparallel diode may be used as switches 206-213. An ordinary skill inthe art should appreciate that switches 206-213 may be implemented byother devices.

Further, the power flow controller 200 may comprise a fail-normal switch214. The fail-normal switch 214 may be connected across the BTBconverter 230. In one embodiment, the fail-normal switch 214 may becoupled to the tap 240 of the transformer 201. In various embodiments,the fail-normal switch 214 may be realized by two anti-parallelthyristors. In further embodiments, the fail-normal switch 214 may berealized by electromechanical or vacuum switches in parallel with thethyristors. The fail-normal switch 214 provides fast response. In casesof a converter failure or a line side fault, the fail-normal switch 214turns on and bypasses the transformer 201 and the BTB converter 230 ofthe power flow controller 200, which avoids single-point failures andincreases the system reliability. As system faults may result incurrents of the order of 10-20 kA for duration of 5-10 cycles beforebeing interrupted by protective mechanism. When a fault is detected, theBTB converter 230 is switched off and the fail-normal switch 214 isturned on such that the fault current flows through the fail-normalswitch 214. The DC capacitor 205 of the BTB converter 230 only needs tohandle the fault current for a short delay (10-20 micro seconds) betweenthe fault detection and the fail-normal switch turn on time. In someembodiments where the transformer 201 is a load tap changing (LTC)transformer, the fail-normal switch 214 retains the passive transformerfunctionality.

The BTB converter 230 is a fractionally rated BTB converter as the inputvoltage V_(S) of the BTB converter 230 is a fraction of the line voltageV₁. For example, the input voltage V_(S) of the BTB converter 230 may beless than 10% of the rated voltage of the transformer 201. In variousembodiments, the BTB converter 230 may be coupled between +/−10% taps ofthe transformer 201. In turn, the rating of the BTB converter 230 isonly a fraction (e.g., equal or less than 10%) of the total controlledpower rating as the switches 206-213 handle only a fraction of thetransformer 201 rated voltage. Since the BTB converter 230 achieves thefractional rating because of the fractional voltage rather than thecurrent, multi-level converter may be implemented in various embodimentswhere series operation of the switches cannot be avoided.

FIG. 3 illustrates an exemplary schematic diagram of a single-phase3-level power flow controller 300 in accordance with an embodiment ofthe invention. The example power flow controller 300 comprises atransformer 301 and a 3-level BTB converter 340. In one embodiment, thetransformer 301 may be an autotransformer. In one embodiment, thetransformer 301 may be an isolated step-down transformer. The BTBconverter 340 comprises a transformer side converter (TSC) 302 which isan AC-DC converter, a direct-current (DC) link 303, a line sideconverter (LSC) 304 which is a DC-AC converter, input filter inductors315-316, and output filter inductors 317-318 and capacitor 319. Invarious embodiments, the LSC 304 may be a full-bridge or a half-bridgeDC-AC inverter. The TSC 302 and the LSC 304 are coupled through thecommon DC link 303. In the illustrated example, the common DC link 303is a capacitor link comprising two capacitors 305 and 306. The TSC 302comprises switches 320-327 and the LSC comprises switches 328-335. TheTSC 302 comprises legs 341 and 342: the leg 341 comprises switches320-323 and the leg 342 comprises switches 324-327. The legs 341 and 342of the TSC 302 may be coupled between two taps of the transformer 301.The legs 341 and 342 of the TSC 302 may be coupled between the taps +/−nof the transformer 301, respectively. Further, the LSC 304 compriseslegs 343 and 344: the leg 343 comprises switches 328-331 and the leg 344comprises switches 332-335. The legs of the LSC 304 may be coupled tothe output of the BTB converter 340, either directly or indirectly.

The power flow controller 300 may be installed between two AC twosources. The power flow controller 300 may be connected in series with atransmission line. In various embodiments, taps of the transformer 301may be floating around the voltage level of the transmission line wherethe power flow controller is installed. The input of the power flowcontroller 300 is connected to one node of a transmission line and theoutput of the power flow controller 300 is connected to another node ofthe transmission line. In various embodiments, the mid-point of the BTBconverter 340 may serve as the input of the power flow controller 300.In further embodiments, the tap 360 of the transformer 301 may serve asthe input of the power flow controller 300. Taps of the transformer 301may be floating around the voltage level at the tap 360 of thetransformer 301, which may be the line voltage. The legs 341 and 342 ofthe TSC 302 may be coupled between taps 361 and 362 of the transformer301, respectively, either directly or indirectly. The tap 360 may be atap that is located between the taps 361 and 362. In one embodiment, thetaps 361 and 362 are +/−n of the transformer 301, and the tap 360 is thecenter tap of the tap winding (i.e., +/−n) of the transformer 301. Asillustrated, the tap 360 of the transformer 301 may be connected to themid-point of the LSC 304. In one embodiment, the center tap of the tapwinding of the transformer 301 may be connected to mid-point of the LSC304 to reduce the loss of the multiple-level BTB converter 340 includingthe switching/conduction and passive losses and to reduce overheating ofthe capacitors comprised in the DC link as this neutral line allowscurrent to flow directly from the input of the BTB converter to theoutput of the BTB converter. In various embodiments, the output of thepower flow controller 300 is the output of the BTB converter 340. Theoutput of the power flow controller 300 is coupled to another node ofthe transmission line.

The BTB converter 340 is a fractionally rated BTB converter as the inputvoltage V_(S) of the BTB converter 340 is a fraction of the line voltageV₁. For example, the input voltage V_(S) of the BTB converter 340 may beless than 10% of the rated voltage of the transformer 301. In variousembodiments, the BTB converter 340 is connected between +/−10% taps ofthe transformer 301. In turn, the rating of the BTB converter 340 isonly a fraction (e.g., equal or less than 10%) of the total controlledpower rating as the switches 320-335 handle only a fraction of thetransformer 301 rated voltage.

Still referring to FIG. 3, the control module 350 regulates the switches320-335. In various embodiments, the control module 350 may generateswitching pulses to regulate the turn-on and turn-off of each switch. Insome embodiments, the control module 350 may interact with the gatedrivers for switches 320-335. In the illustrated example, the switches320-335 are two-quadrant switches that are implemented by an IGBT and ananti-parallel diode. In some embodiments, metal-oxide-semiconductorfield-effect transistor (MOSFETs) with an antiparallel diode may be usedas switches 320-335. An ordinary skill in the art would appreciate thatswitches 320-335 may be implemented by other devices.

Further, the power flow controller 300 may comprise a fail-normal switch315. The fail-normal switch 315 may be connected across the BTBconverter 340. In one embodiment, the fail-normal switch 315 may becoupled to the tap 360 of the transformer 301. In various embodiments,the fail-normal switch 315 is realized by two anti-parallel thyristors.In further embodiments, the fail-normal switch 315 may be realized byelectromechanical or vacuum switches in parallel with the thyristors.The fail-normal switch 315 provides fast response. In cases of aconverter failure or a line side fault, the fail-normal switch 315 turnson and bypasses the transformer 301 and the BTB converter 340 of thepower flow controller 300, which avoids single-point failures andincreases the system reliability. As system faults may result incurrents of the order of 10-20 kA for duration of 5-10 cycles beforebeing interrupted by protective mechanism. When a fault is detected, theBTB converter 340 is switched off and the fail-normal switch 315 isturned on such that the fault current flows through the fail-normalswitch 315. The DC capacitors 305-306 of the BTB converter 340 only needto handle the fault current for a short delay (10-20 micro seconds)between the fault detection and the fail-normal switch turn on time. Insome embodiments where the transformer 301 is a load tap changing (LTC)transformer, the fail-normal switch 315 retains the passive transformerfunctionality.

FIGS. 4A and 4B illustrate principles of operation of variousembodiments of the power flow controllers with BTB converters asdescribed herein. FIG. 4A is a diagram illustrating a system with aninstallation of a power flow controller 406 in accordance with anembodiment of the power flow controllers with BTB converters asdescribed herein. FIG. 4B is a vector diagram illustrating principles ofoperation of a power flow controller in accordance with an embodiment ofthe invention. The exemplary system 400 comprises two generators 401 and403, two buses 402 and 404, and a transmission line 405. V₁ is thevoltage at Bus 402, and V₂ is the voltage at Bus 404. In the illustrateexample, the power flow controller 406 is installed in series with thetransmission line 405. The power flow controller 406 performs dynamicpower flow control of both active and reactive power of the power system400. Such dynamic power flow control is achieved by actively controllingthe phase and magnitude of the transfer voltage in a certain range. TheBTB converter 407 synthesizes the converter input voltage V_(S) togenerate a voltage V_(CONV) that may be of different magnitude and phasecompared to V_(S). As a result, the phase and magnitude of the outputvoltage V_(out), resultant of Bus 1 Voltage V₁ and V_(CONV), may becontrolled to achieve both active and reactive power control.

Referring to FIG. 4B, as illustrated, the initial phase differencebetween bus 402 voltage V₁ and bus 404 voltage V₂ is δ. The power flowcontroller 406 inserts a voltage V_(CONV) to V₁, which creates theoutput voltage V_(out). The output voltage V_(out) and the Bus 402voltage V₁ may have different phases and amplitudes. The amplitude ofthe output voltage V_(out) may be adjusted by adjusting the amplitudeand phase angle of the inserted voltage V_(CONV). Further, the phasedifference between the output voltage V_(out) and the Bus 404 voltage V₂is (δ+φ), which may be adjusted by adjusting the amplitude and phaseangle of the inserted voltage V_(CONV). As such, control of both activepower and reactive power is achieved as the active power transferredbetween buses 402 and 404

$\left( {{P = {\frac{V_{out}V_{2}}{X_{Line}}\sin \; \left( {\delta + \varphi} \right)}},} \right.$

where X_(Line) is the line impedance) is a function of (δ+φ), and thereactive power transferred between buses 402 and 404

$\left( {{Q = {\frac{V_{out}V_{2}}{X_{Line}}\left( {{\cos \left( {\delta + \varphi} \right)} - \frac{V_{out}}{V_{1}}} \right)}},} \right.$

where X_(Line) is the line impedance) is a function of the voltageamplitude V_(out) and V₂.

The series voltage V_(CONV) that the power flow controller can generateis a function of the input voltage V_(S), which in turn depends on thetransformer taps across which the BTB converter is connected. As shown,the range of the voltage V_(CONV) is a circle 410 with a radius of

$\frac{V_{S}}{2}.$

The power flow control range of the power flow controller is a functionof the input voltage V_(S), line impedance X_(Line), and the phasedifference δ between the sending end voltage V₁ and the receiving endvoltage V₂. The active power P, the sending end reactive power at Bus402 Q₁, and the receiving end reactive power at Bus 404 Q₂ may beexpressed in Equations (1), (2), and (3) respectively:

$\begin{matrix}{P = {\frac{V_{out}V_{2}}{X}\sin \; \left( {\delta + \varphi} \right)}} & (1) \\{Q_{1} = {\frac{V_{out}}{X}\left( {V_{out} - {V_{2}{\cos \left( {\delta + \varphi} \right)}}} \right)}} & (2) \\{{Q_{2} = {\frac{V_{out}}{X}\left( {V_{2} - {V_{out}\cos \; \left( {\delta + \varphi} \right)}} \right)}}{{{{wher}e}\mspace{14mu} V_{out}} = {{\sqrt{V_{1}^{2} + V_{CONV}^{2}}\varphi} = {\tan^{- 1}\frac{V_{CONV}\sin \; \theta}{V_{1} + {V_{CONV}\cos \; \theta}}}}}} & (3)\end{matrix}$

Further, referring back to FIG. 4A, the power flow controller 406 alsohas shunt VAR capability. The shunt VAR range is the same as the BTBconverter 407 rating. The range of the shunt VAR is given in Equation(4):

Q _(SHUNT)=2n*√{square root over (3)}*V _(Bus) *I _(CONV) _(_) _(rating)  (4)

where V_(Bus) is the line-to-line voltage, n is the transformer tapratio, and I_(CONV) _(_) _(rating) is the current rating of the BTBconverter.

As illustrated, various embodiments have circular range of operation.For a converter with input voltage V_(S) and the DC link voltage V_(DC),the fundamental voltage that the converter may synthesize is given byEquation (5):

$\begin{matrix}{V_{CONV} = {{\frac{V_{s,{PEAK}}V_{DC}}{2}\left( {{k_{q}\sin \; \theta} + {k_{p}\cos \; \theta}} \right)\mspace{14mu} {such}\mspace{14mu} {that}\mspace{14mu} \sqrt{\left( {k_{q} - 0.5} \right)^{2} + k_{p}^{2}}} < 0.5}} & (5)\end{matrix}$

where V_(S,PEAK) is the peak voltage of the converter input voltage,k_(q) is the reactive power coefficient, and k_(p) is the active powercoefficient.

The in-phase component k_(q) sin θ of the converter voltage controls thereactive power flowing through the line where the power flow controlleris deployed while the out of phase component k_(p) cos θ controls theactive power.

FIGS. 5A-C illustrate simulation waveforms of an embodiment of theinvention as described herein. One embodiment of a power flow controlleris simulated in a 2-bus 138 kV system with parameters shown in Table 1.The converter parameters are given in Table 2. The control parametersare designed using standard bode plot techniques and are given in Table3.

TABLE 1 138 kV Test System Parameters Parameter Value Voltage 138 kVL-L, 80 kV L-G Line 30 miles, 0.168 + j0.79 ohms/mile Taps (n) +/−5%

TABLE 2 Converter Parameters Parameter Value V_(S) 8.0 kV V_(DC) 12.5 kVC_(DC) 1 mF L_(diff) 4 mH L_(f) 1 mH C_(f) 50 microF Δ 2 degrees

TABLE 3 Converter Control Parameters Parameter Value Differential ModeV_(DC) (base) 12.5 kV I_(diff) (base) 500 A K_(p) k_(i) (voltage loop)4, 10 respectively K_(p) k_(i) (current loop) 0.05, 0.5 respectivelyCommon Mode V_(CONV) (base) 4 kV I_(line) (base) 800 A K_(p) k_(i) 0.1,0.5 respectively

Referring to FIG. 5A, when δ is 2 degrees, the power flow in the linewithout the power flow controller is 28 MW. The power flow controllermay vary power flow from 66 MW to −10 MW, thereby providing a controlrange of 38 MW while maintaining the reactive power constant. Referringto FIG. 5B, the converter input voltage V_(S), line current andconverter voltage V_(CONV) at maximum active power range are shown. Themaximum V_(CONV) that the converter can generate

$\frac{V_{S}}{2}.$

Referring to FIG. 5C, the differential current being controlled toregulate the DC bus voltage is shown. A controllability range of +/−38MW/MVAR is achieved with a converter rating of 10% of the power beingcontrolled. As the converter is connected between +/−5% taps, theconverter only handles peak voltage of 12 kV.

FIGS. 6A-C depict the control block diagrams in various embodiments ofthe invention, which may be implemented by a computing module asillustrated in FIG. 7. FIG. 6A depicts the common mode and differentialmode control in various embodiments of the invention as describedherein. In the illustrated example, the LSC 604 is a half-bridge DC-ACconverter. In various embodiments, the power flow in the power flowcontroller comprises two components: a common mode and a differentialmode. The common mode is the primary component controlling the linecurrent while the differential component is used for auxiliary purposessuch as controlling the DC link 603 voltage.

The common mode power is controlled by the LSC 604 to generate requisitepower flow in the line. The common mode converter voltage V_(comm) isregulated controlled to regulate the common mode current I_(Comm), whichis the same as the line current. The LSC 604 may generate either

$\frac{V_{S} + V_{DC}}{2}\mspace{14mu} {or}\mspace{14mu} \frac{V_{S} - V_{DC}}{2}$

depending on the status of the switches 610 and 611 of the LSC 604. Anyvoltage waveform that remains within the range of

$\frac{V_{S} + V_{DC}}{2}{and}\frac{V_{S} - V_{DC}}{2}$

may be synthesized by regulating the duty cycle of the switches 610 and611 of the LSC 604. Accordingly, the maximum fundamental voltage thatmay be generated by the converter with respect to the center tap of thetransformer 601 has a peak voltage of

$\frac{V_{DC}}{2}.$

In some embodiments, V_(DC) is controlled to be the peak of V_(S).

The differential power component shuffles energy between the transformer601 and the DC bus through the TSC 602. The active component of thedifferential mode current I_(Diff) is controlled to regulate the mean DCcapacitor 609 voltage and the reactive component is controlled toregulate the shunt VAR of the power flow controller.

FIGS. 6B-D illustrate the control block diagrams in various embodimentsof the invention as described herein. The control schemes comprise acommon mode control scheme and a differential mode control scheme, whichmay be implemented by various control modules of different embodiments,for example, the control module 220 as illustrated in FIG. 2 and thecontrol module 350 as illustrated in FIG. 3.

FIG. 6B illustrates the common mode control block diagram in variousembodiments of the invention as described herein. In one embodiment, thecommon mode control is realized in a current loop. For desired activepower and series VAR control, in various embodiments, the control modulesets the current references for the common mode control. The control isachieved in d-q synchronous reference frame. The desired line currentsI_(comm) ^(ref) are compared with the actual current I_(comm), and theerror is propagated through the PI controller, which in turn generatesconverter output voltage V_(comm) ^(ref). The reference voltage is thencompared with the carrier wave to generate switching pulses for the LSCof a power flow controller.

FIG. 6C illustrates a differential mode control block diagram in variousembodiments of the invention as described herein. In one embodiment, thecommon mode control is realized in a voltage loop as the objective ofthe differential mode control is to maintain the average DC link voltageV_(DC,0) at a desired value and also control the shunt VAR. The averagevalue of the DC link voltage is extracted with a low pass filter andthen compared with the reference value. The voltage error is fed to thePI regulator which in turn generates the TSC reference currentI_(diff,d) ^(ref). The control generates the reference for the reactivecomponent of the differential current I_(diff,q) ^(ref), as per theshunt VAR requirements. The differential currents are regulated bygenerating appropriate switching pulses for the TSC of a power flowcontroller.

FIG. 6D illustrates a differential mode control block diagram in variousembodiments of the invention as described herein. In variousembodiments, the reactive power flow across the capacitor of the DC linkmay introduce low frequency ripples such as a 2^(nd) harmonic voltageripple across the capacitor. By achieving power balance between inputand output of the converter, the low frequency ripples and the capacitorsize may be minimized. Instantaneous power balance between the input andthe output of the BTB converter of a power flow controller may beachieved by introducing controlled harmonics, for example, 3^(rd) and5^(th) harmonic in the input side of the BTB converter, therebyminimizing instantaneous power imbalance on either side of theconverter. The input power available at the harmonic frequency togetherwith the available power at the fundamental frequency will try tobalance the fundamental power required on the output of the converter ateach instant. In various embodiments, the DC link capacitor may bereduced by this control scheme. As the induced harmonics on the inputare controlled, the peak currents through the switches of the BTBconverter are limited. Further, no harmonics are induced in the linecurrent.

As illustrated, the in-phase component V_(dc) _(_) _(LSC,2d) and theout-of-phase component V_(dc) _(_) _(LSC,2q) of the 2^(nd) harmonicripple flowing through the DC link of a BTB converter caused by thecommon mode current may be determined from the current I_(LSC) and thevoltage V_(LSC) of the LSC of a BTB converter. The in-phase componentV_(dc) _(_) _(LSC,2d) may be compensated by the differential modereactive current component which in turn is controlled by the q-axis TSCvoltage V_(TSC,1q). The out-of-phase component V_(dc) _(_) _(LSC,1q) maybe compensated by the 3^(rd) harmonic current in the differential mode,which is controlled by the 3^(rd) harmonic voltage V_(TSC,3q) of theTSC. The fundamental and the 3^(rd) harmonic reference voltages aresummed up and compared with the carrier wave to generate switchingpulses for the TSC of a BTB converter. In various embodiments, thecarrier wave is a pulse-width modulation (PWM) carrier wave.

As used herein, the term set may refer to any collection of elements,whether finite or infinite. The term subset may refer to any collectionof elements, wherein the elements are taken from a parent set; a subsetmay be the entire parent set. The term proper subset refers to a subsetcontaining fewer elements than the parent set. The term sequence mayrefer to an ordered set or subset. The terms less than, less than orequal to, greater than, and greater than or equal to, may be used hereinto describe the relations between various objects or members of orderedsets or sequences; these terms will be understood to refer to anyappropriate ordering relation applicable to the objects being ordered.

As used herein, the term module might describe a given unit offunctionality that can be performed in accordance with one or moreembodiments of the present invention. As used herein, a module might beimplemented utilizing any form of hardware, software, or a combinationthereof. For example, one or more processors, controllers, ASICs, PLAs,PALs, CPLDs, FPGAs, logical components, software routines or othermechanisms might be implemented to make up a module. In implementation,the various modules described herein might be implemented as discretemodules or the functions and features described can be shared in part orin total among one or more modules. In other words, as would be apparentto one of ordinary skill in the art after reading this description, thevarious features and functionality described herein may be implementedin any given application and can be implemented in one or more separateor shared modules in various combinations and permutations. Even thoughvarious features or elements of functionality may be individuallydescribed or claimed as separate modules, one of ordinary skill in theart will understand that these features and functionality can be sharedamong one or more common software and hardware elements, and suchdescription shall not require or imply that separate hardware orsoftware components are used to implement such features orfunctionality.

Where components or modules of the invention are implemented in whole orin part using software, in one embodiment, these software elements canbe implemented to operate with a computing or processing module capableof carrying out the functionality described with respect thereto. Onesuch example computing module is shown in FIG. 8. Various embodimentsare described in terms of this example-computing module 800. Afterreading this description, it will become apparent to a person skilled inthe relevant art how to implement the invention using other computingmodules or architectures.

Referring now to FIG. 7, computing module 700 may represent, forexample, computing or processing capabilities found within desktop,laptop and notebook computers; hand-held computing devices (PDA's, smartphones, cell phones, palmtops, etc.); mainframes, supercomputers,workstations or servers; or any other type of special-purpose orgeneral-purpose computing devices as may be desirable or appropriate fora given application or environment. Computing module 700 might alsorepresent computing capabilities embedded within or otherwise availableto a given device. For example, a computing module might be found inother electronic devices such as, for example, digital cameras,navigation systems, cellular telephones, portable computing devices,modems, routers, WAPs, terminals and other electronic devices that mightinclude some form of processing capability.

Computing module 700 might include, for example, one or more processors,controllers, control modules, or other processing devices, such as aprocessor 704. Processor 704 might be implemented using ageneral-purpose or special-purpose processing engine such as, forexample, a microprocessor, controller, or other control logic. In theillustrated example, processor 704 is connected to a bus 702, althoughany communication medium can be used to facilitate interaction withother components of computing module 700 or to communicate externally.

Computing module 700 might also include one or more memory modules,simply referred to herein as main memory 708. For example, preferablyrandom access memory (RAM) or other dynamic memory, might be used forstoring information and instructions to be executed by processor 704.Main memory 708 might also be used for storing temporary variables orother intermediate information during execution of instructions to beexecuted by processor 704. Computing module 700 might likewise include aread only memory (“ROM”) or other static storage device coupled to bus702 for storing static information and instructions for processor 704.

The computing module 700 might also include one or more various forms ofinformation storage mechanism 710, which might include, for example, amedia drive 712 and a storage unit interface 720. The media drive 712might include a drive or other mechanism to support fixed or removablestorage media 714. For example, a hard disk drive, a floppy disk drive,a magnetic tape drive, an optical disk drive, a CD or DVD drive (R orRW), or other removable or fixed media drive might be provided.Accordingly, storage media 814 might include, for example, a hard disk,a floppy disk, magnetic tape, cartridge, optical disk, a CD or DVD, orother fixed or removable medium that is read by, written to or accessedby media drive 712. As these examples illustrate, the storage media 714can include a computer usable storage medium having stored thereincomputer software or data.

In alternative embodiments, information storage mechanism 710 mightinclude other similar instrumentalities for allowing computer programsor other instructions or data to be loaded into computing module 700.Such instrumentalities might include, for example, a fixed or removablestorage unit 722 and an interface 720. Examples of such storage units722 and interfaces 720 can include a program cartridge and cartridgeinterface, a removable memory (for example, a flash memory or otherremovable memory module) and memory slot, a PCMCIA slot and card, andother fixed or removable storage units 722 and interfaces 720 that allowsoftware and data to be transferred from the storage unit 722 tocomputing module 700.

Computing module 700 might also include a communications interface 724.Communications interface 724 might be used to allow software and data tobe transferred between computing module 700 and external devices.Examples of communications interface 724 might include a modem orsoftmodem, a network interface (such as an Ethernet, network interfacecard, WiMedia, IEEE 802.XX or other interface), a communications port(such as for example, a USB port, IR port, RS232 port Bluetooth®interface, or other port), or other communications interface. Softwareand data transferred via communications interface 724 might typically becarried on signals, which can be electronic, electromagnetic (whichincludes optical) or other signals capable of being exchanged by a givencommunications interface 724. These signals might be provided tocommunications interface 724 via a channel 728. This channel 728 mightcarry signals and might be implemented using a wired or wirelesscommunication medium. Some examples of a channel might include a phoneline, a cellular link, an RF link, an optical link, a network interface,a local or wide area network, and other wired or wireless communicationschannels.

In this document, the terms “computer program medium” and “computerusable medium” are used to generally refer to media such as, forexample, memory 708, storage unit 720, media 714, and channel 728. Theseand other various forms of computer program media or computer usablemedia may be involved in carrying one or more sequences of one or moreinstructions to a processing device for execution. Such instructionsembodied on the medium, are generally referred to as “computer programcode” or a “computer program product” (which may be grouped in the formof computer programs or other groupings). When executed, suchinstructions might enable the computing module 700 to perform featuresor functions of the present invention as discussed herein.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not of limitation. Likewise, the various diagrams maydepict an example architectural or other configuration for theinvention, which is done to aid in understanding the features andfunctionality that can be included in the invention. The invention isnot restricted to the illustrated example architectures orconfigurations, but the desired features can be implemented using avariety of alternative architectures and configurations. Indeed, it willbe apparent to one of skill in the art how alternative functional,logical or physical partitioning and configurations can be implementedto implement the desired features of the present invention. Also, amultitude of different constituent module names other than thosedepicted herein can be applied to the various partitions. Additionally,with regard to flow diagrams, operational descriptions and methodclaims, the order in which the steps are presented herein shall notmandate that various embodiments be implemented to perform the recitedfunctionality in the same order unless the context dictates otherwise.

Although the invention is described above in terms of various exemplaryembodiments and implementations, it should be understood that thevarious features, aspects and functionality described in one or more ofthe individual embodiments are not limited in their applicability to theparticular embodiment with which they are described, but instead can beapplied, alone or in various combinations, to one or more of the otherembodiments of the invention, whether or not such embodiments aredescribed and whether or not such features are presented as being a partof a described embodiment. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, can be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

1. An apparatus for controlling real and reactive power flow between afirst AC source and a second AC source, comprising: a back-to-backconverter comprising an AC-DC converter coupled between a first tap anda second tap of a transformer, the AC-DC converter comprising a firstset of switches, a DC-AC converter coupled to the second AC source, theDC-AC converter comprising a second set of switches, and a DC linkcoupled to the AC-DC converter and the DC-AC converter, wherein a thirdtap of the transformer is coupled to the first AC source, and an inputvoltage to the back-to-back converter is less than a voltage at thethird tap of the transformer.
 2. The apparatus of claim 1, wherein aninput voltage to the back-to-back converter is a fraction of the voltageat the third tap of the transformer.
 3. The apparatus of claim 1,wherein the transformer further comprises a set of taps floating aroundthe voltage at the third tap of the transformer.
 4. The apparatus ofclaim 1, wherein the transformer is an auto transformer or an isolatedstep-down transformer.
 5. The apparatus of claim 1, further comprisingthe transformer.
 6. The apparatus of claim 1, wherein the third tap islocated between the first tap and the second tap.
 7. The apparatus ofclaim 1, wherein the AC-DC converter comprises a first leg and a secondleg, the first leg coupled to the first tap of the transformer and thesecond leg coupled to the second tap of the transformer.
 8. Theapparatus of claim 1 having an output, wherein the DC-AC convertercomprises a first leg and a second leg, the first leg and the second legcoupled to the output of the apparatus.
 9. The apparatus of claim 1,further comprising a control module, wherein the control modulegenerates a common mode control signal such that the common mode currentis within a first predetermined value and generates a differential modecontrol signal such that a differential current is within a secondpredetermined value.
 10. The apparatus of claim 9, wherein the controlmodule generates a first set of switching pulses to the first set ofswitches according to the differential mode control signal, and a secondset of switching pulse to the second set of switches according to thecommon mode control signal.
 11. The apparatus of claim 1, wherein thefirst and the second set of switches are two-quadrant switches.
 12. Theapparatus of claim 1, wherein the DC-AC inverter is a multiple levelinverter.
 13. The apparatus of claim 1 having an output, furthercomprising: an input filter coupled to the transformer and the AC-DCconverter; and an output filter coupled to the DC-AC converter and theoutput of the apparatus.
 14. The apparatus of claim 1, wherein the DClink comprises a capacitor.
 15. The apparatus of claim 1, furthercomprising a fail-normal switch, wherein the fail-normal switch iscoupled across the back-to-back converter.